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| Stainless Oscillating Pistons? |
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| Stilldrillin:
--- Quote from: Darren on April 21, 2009, 05:33:30 AM ---Did the PTFE piston expand with steam or were you talking about the brass one? So you pinned the piston then, that's good info to know :thumbup: --- End quote --- Darren, This plastic piston lark isn`t quite straightforward........ ::) The engine ran perfectly at room temp, on air. Steam heat expanded the piston, locking it solid, as far as the engine was concerned. :scratch: Piston into lathe, take off "half a thou" per side. Back onto engine....... Several times! The lathe chuck is "true"....... But not that good, so the piston is not truly round now. Tried a little `speriment this morning. Dunked a couple of cylinders & machined ptfe into a mug of boiling water. Got it to slide in one cyl. But not in the other (which has a thinner cyl wall). Measured o/d when cooled....... It`s only .001" or so below std (.3125")!!! :bugeye: I get different readings from mic & vernier. Also depends weather the sun`s shining or not....... :bang: I think I`ll have to steam individual piston/ cyl sets before mounting to the 3cyl engine...... :scratch: The piston s are tapped 1/8 Whit & screwed onto the rod, with a spot of loctite. David D |
| sbwhart:
David One Little trick we use to do with nylon parts was to boil them, just like an egg, many plastics take up moisture and grow, I know that with nylon it can take up to about 1% of its own weight, don't know what it is for PTFE but you could try it before you machine to size. Cheers Stew |
| Stilldrillin:
--- Quote from: sbwhart on April 21, 2009, 08:13:19 AM ---David One Little trick we use to do with nylon parts was to boil them, just like an egg, many plastics take up moisture and grow, I know that with nylon it can take up to about 1% of its own weight, don't know what it is for PTFE but you could try it before you machine to size. Cheers Stew --- End quote --- Sounds good to me Stew! :clap: I`ll try anything....... :thumbup: The used plastic piston measures around .309". The boiling water plug measures around .312". I guess steam is hotter than boiled water...... And it shows in the differing diameters! David D |
| Darren:
I did mention plastics absorb water, I didn't think it was that much though ! |
| bogstandard:
So now you lads understand when I go on about the correct materials for making cylinders and pistons out of. That is why I always ask if it is going to be run on just air or steam. It makes a very big difference. Darren, If you are going to make the twin engine for running on steam, I would stay with the recommended metals. David's engine is a lot different in design, and yours will have a lot more heat, friction and stresses involved. You could modify my design to use PTFE piston rings, but I don't think there would be any advantage over the standard o-rings. John |
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